This invention generally relates to transport refrigeration systems, and is specifically concerned with a refrigerant system and method having a combined economizer and engine coolant heat exchanger for increasing the efficiency of both cooling and heating cycles of the system.
Refrigeration systems that utilize economizers to enhance the efficiency of the refrigeration cooling cycle are known in the prior art. Such systems generally employ a refrigerant compressor having a discharge port, a suction port, and an intermediate pressure port. In a transport refrigeration system, the compressor is driven by an internal combustion engine. The discharge and suction ports are connected to a refrigeration circuit that generally comprises a condenser coil for condensing gaseous refrigerant into a liquid, and an evaporator assembly that includes an expansion valve for converting the liquid refrigerant back into a gas, and an evaporator coil that is thermally connected to a conditioned space, which may be a truck trailer. The outlet of the evaporator coil is ultimately connected to the suction port of the compressor. In such systems, the economizer is located downstream from the condenser coil and upstream from the expansion valve of the evaporator assembly. Such economizers are generally formed from a tube and shell heat exchanger in which the tube short circuits a small amount of the refrigerant flowing toward the evaporator assembly to the intermediate suction port of the compressor, while the main flow of refrigerant is conducted through the shell of the heat exchanger. A second expansion valve is placed just upstream of the tube of the economizer tube and shell heat exchanger in order to expand the small volume of liquid refrigerant into gaseous refrigerant which in turn causes the tube to precool the liquid refrigerant flowing through the shell of the economizer. The precooling of the liquid refrigerant prior to its expansion by the main expansion valve of the evaporator assembly advantageously increases the cooling capacity of the system.
Refrigeration systems capable of operating in a heating and defrosting mode are also known which employ heat exchangers to enhance the efficiency of the heating cycle. Such refrigeration systems typically employ a three-way mode valve to divert hot, gaseous refrigerant around the expansion valve of the evaporator assembly and directly into the evaporator coil, thus converting the evaporator coil into a heat radiating condenser for either defrosting or heating applications. In such systems, the heat exchanger is again placed downstream from the condenser coil and upstream from the evaporator coil that is used as a condenser during the heating cycle. The heat exchanger includes a tube for conducting the gaseous refrigerant to the compressor intermediate suction port, and a shell surrounding the tube for conducting a flow of a heating medium around the tube, which may be the hot liquid coolant of the radiator system of the internal combustion engine used to drive the compressor. The additional heat transferred to the gaseous refrigerant from the engine coolant advantageously enhances the heating capacity of the evaporator coil that is converted to use as a heat-radiating condenser during the heating cycle.
Systems utilizing a combined economizer and engine coolant heat exchanger are also known in the prior art in U.S. Pat. No. 4,850,197 assigned to the present assignee. In this combined device, a jacket that may carry hot engine coolant and a coiled tube that may carry expanding refrigerant are surrounded by a single shell which may conduct liquid refrigerant to the evaporator assembly or gaseous refrigerant to the compressor, depending upon whether the refrigerant system is operated in a cooling or heating mode. While such systems have proven to be effective for their intended purpose, the inventors have noted a number of areas where such performance might be improved. For example, the applicants have noted that the direct thermal interface between the tube conducting engine coolant and gaseous refrigerant enroute to the compressor during the heating cycle of this system may create problems when the engine coolant is at a low temperature (as may occur, for example, during start-up conditions when the ambient temperature is below freezing). Under such conditions, the engine coolant actually "bleeds" heat away from the refrigerant instead of contributing heat to it, which in turn can impair the functioning of the heating cycle. Alternatively, in instances where very hot engine coolant is first introduced into such a heat exchanger, the resulting transfer of heat can be so great as to create unduly high pressures in the gaseous refrigerant entering the compressor. Finally, the inventors have noted that it would be desirable if the size of such a combined heat exchanger could be reduced to facilitate a compact arrangement of the refrigeration components in the limited space available in a transport refrigeration system.